BPy add Path in Textfile from Text-Editor
Couple of ways.

Already pointed out hassle re slosh in comments

To include a string in another block string, could use formatting, I like the old and deprecatious % Checkout the python docs for string formatting str.format(...)

text = '''
%s blah 
%s blah
%s blah
''' % (("XXX",) * 3) # all 3 occurences
print(text)


output

XXX blah 
XXX blah
XXX blah


or use a unique TOKEN and replace

print(text.replace("XXX", "YYY"))


output

YYY blah 
YYY blah
YYY blah


Since the parts are paths strongly recommend you lookup in python docs , os.path.join(...).